The present invention relates to molding media and materials, and particularly to a molding medium for use in the evaporative pattern casting process, and even more particularly, to a free flowing molding medium for use in the evaporative pattern casting process which does not require a refractory coating to be applied to the evaporative pattern. The invention further relates to an evaporative pattern casting process wherein free flowing molding material is used and wherein the pattern is not coated with a refractory material.
In the evaporative pattern casting process, a form or pattern, generally comprising polystyrene foam, of the item to be cast is made. The foam pattern is placed in a pouring box and embedded in a molding material. A foam leader leads from the pattern to the upper surface of the molding material, providing a passageway for the molten metal. Molten metal is then poured into the pouring box, with the result that the molten metal evaporates the pattern, thus displacing it. The metal is allowed to cool and the cast item can be removed from the pouring box once it has cooled. See, e.g., U.S. Pat. No. 2,830,343 to Shroyer.
In a further refinement of the evaporative pattern casting method, the molding material is unbonderized and free flowing. The free flowing material is poured into the pouring box and compacted so as to completely surround the foam pattern and the leader. The molten metal is then poured into the box, and it has been theorized that, upon contact with the cooler molding material, the polystyrene evaporated by contact with the molten metal will condense and thus retain the unbonded molding material in position a sufficient length of time to support the entering molten metal displacing the pattern. See, e.g. U.S. Pat. No. 3,157,924 to Smith. Experiments have indicated, however, that it is the formation of gases due to the evaporation of the foam pattern that allows the unbonded molding material to remain in position.
The evaporative pattern process has great potential to be adopted widely in the foundry industry as an economical and environmentally safe casting production process. To date, however, this potential has not been fully realized because of the present method and materials that are used for moldings.
Presently, to produce a casting with an acceptable reliability and quality using the evaporative pattern casting process, the following steps are required after the successful production and assembly of the disposable pattern:
1. A so-called wash is produced and applied uniformly over the surfaces of the evaporative pattern. The "wash" can be as described in U.S. Pat. Nos. 2,701,902, 2,829,060, 3,498,360, 3,314,116, 3,169,288, 3,351,123, or 3,270,382, British Patent No. 1,281,082, or many other different proprietary brands which all have one thing in common: a finely ground refractory material such as aluminum, zirconium or silica flour is emulsified and suspended in a carrying agent, the most commonly used such material being water or alcohol.
2. This coating material, after its application onto the pattern, then has to be dried. As the result of the evaporation of the water or alcohol or the setting up of the carrying agent, a thin shell is produced around the pattern, coating all surfaces of the evaporative pattern.
3. The dried and coated pattern is inserted or invested into a dry free-flowing molding material such as silica sand of a specific grain fineness disposed in a pouring box.
4. During the investment of the pattern into the molding medium, the molding medium is either aerated, using air or other gas, or vibrated to reduce the angle of repose of the sand close to 0.degree., thus allowing the sand to flow into and fill all areas and inner and outer cavities of the pattern. By angle of repose is meant the angle of a cone formed by pouring the molding medium onto a flat surface. The lower the angle, the closer the material is to a liquid, which essentially takes the shape of the container into which it is poured.
5. The sand then is densified or compacted to provide support for the weight of the liquid metal to be poured into the pouring box.
6. A weight or other blockage means is placed on the top of the molding medium in the pouring box.
7. The mold is filled with liquid metal, thus evaporating the pattern.
8. After the liquid metal has solidified, the weight is removed and the casting and sand are dumped out of the pouring box.
9. The casting is then sent to the cleaning room to be cleaned and readied for shipment.
With the above described procedure, castings of good quality can be produced at present. There are, however, a number of problems remaining with the technique described above. Some problems, for example, are in the areas of the finished casting quality and economics. The refractory coated pattern, depending on the thickness of the coating, will produce a casting which will also be coated with the refractory material which adheres to the molten metal. Since the refractory material is made up of fine particles and these particles tend to cling together, their removal is quite critical, especially for castings that are used for internal combustion engines such as engine blocks or cylinder heads. Any particle which is not removed will then stay in the cooling system and may eventually destroy the coolant pump or its seal or clog up the coolant system radiator. In other areas it may become mixed with the engine lubricant, in which instance it may lead to premature engine wear or failure.
In addition, the coating of the pattern and the drying operation is costly and energy intensive and affects the quality of the casting. Furthermore, the molding medium used with these coating materials is usually dry free-flowing silica sand, which is not environmentally safe since it contains free silica. Additionally, the angle of repose of such sand is around 35.degree. and when compacted it can reach 45.degree.. This angle of repose affects, to a great extent, the ability of the molding medium to fill in the internal cavities, etc. without manual intervention. This is in large part due to the creation of differential pressures in the molding material because the large angle of repose prevents the molding material from behaving like a liquid and generating essentially a uniform pressure in all areas of the interface between the pattern and molding medium. As a result, in some areas of the pattern-molding medium interface, sufficient pressures will not be developed against the pattern to keep the molding medium in place when the molten metal enters the mold, thus causing imperfect castings.
Another effect is that of shrinkage of the molding medium. For example, sand, when compacted, can reduce its volume by as much as 20%. This again hinders some of the ability of the molding medium to properly fill in the inner cavities of a disposable pattern. Due to the shrinkage of the sand as a result of the random grain structure, deformation of the flexible foam pattern may occur, again resulting in imperfect castings. To counter this, the conventional approach has been to apply a heavier refractory coating to the pattern to protect the pattern and/or to reduce the amount of compaction. Both of these measures, however, may result in considerable inacuracy in the finished casting and with respect to the application of a heavier coating, increased drying times and cost.
Although the above problems must be dealt with when using the evaporative pattern casting process, good castings can be produced with this process if the necessary precautions are followed and steps taken.